Butterfly valve having resilient sealing means



July 2, 1963 J, T- FISHER 3,096,071

BUTTERFLY VALVE HAVING RESILIENT SEALING MEANS Filed May 16, 1960 2Sheets-Sheet l f /fg l ffii/,w w s in s.

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/f/ L/ INVENTOR Jlllb Z'EL'JWF ATTORNEY July 2, 1963 J. T. FISHER3,096,071

BUTTERFLY VALVE HAVING RESILIENT SEALING MEANS Filed May 16, 1960 2Sheets-Sheet 2 ATTORNEY 3,096,671 BUTTERFLY VALVE HAVING RESILlENTSEALENG MEANS John T. Fisher, indianapolis, Ind., assigner to Stewart-Warner Corporation, Chicago, El., a corporation of Virginia Filed May16, 1960, Ser. No. 29,537 2 Claims. (Cl. 251-306) This invention relatesto a buttery valve for controlling the flow of cryogenic fluids in aconduit under extremely low temperature conditions and more particularlyto structural modiications of such a valve with a ilexible seal whichparticularly adapt it for eicient operation under these low temperatureconditions.

The transportation of cryogenic iluids at extremely low temperaturesfrom one place to another through a conduit is most often accomplishedintermittently. This being the case, the `temperature extremes to Whichthe transportation apparatus is subjected 4is quite severe. Oneparticularly vital part of the transportation apparatus which isaffected by these temperature extremes is the means for controlling theflow of the low temperature duid, usually taking the form of a valve. Albutterfly valve seems particularly appropriate. IIn such a valve thevalve seat is mounted on the interior of the conduit and detines anopening. In this opening is mounted a disc member adapted to rotate froma full open or inline position within the conduit to a position at rightangles thereto for closing off lthe ow of the liqueed gas. ln order toimprove the engagement between the valve and the valve seat to overcomethe normally deleterious effects of wide temperature variations, certainmodications of the valve and valve seat are proposed to this invention.These modifications substantially eliminate leaks which might occur:between the Valve and the valve seat and undesirable =rictional loadswhich might occur between these elements, both of which yfactors arelargely attributable to the difference in coefcient of expansion betweenmaterials comprising the valve and valve seat and the dilerentproperties they exhibit at extreme low temperatures. Past efforts toeliminate these deliciencies have taken the form of anti-frictionbearings such as ball bearings and the use of a material having someresilience at cryogenic temperatures as the sealing means on one of thevalves sealing surfaces. However, such attempts have not been highlysuccessful because of the fact that Water vapor or other condensatecoming in contact Vwith the bearings freezes as the temperature passesthrough 32 F. to bind the bearing or at least to substantially increasethe friction losses thereof. Further because the use of a resilientmaterial such as Kel F with metal to form the sealing surfaces haspresented such a problem in their different coetdcients of expansionthat proper sealing was di'icult to accomplish. The use of a plasticmaterial and metal tends to scratch or scour the softer materialresulting in leaks. The use of metal as the resilient material alsotends to scratch the sealing surfaces and a metal-to-metal seal willresult in very high frictionafforces to move the butterlly.

It is therefore the primary object of this invention to provide animproved valve structure for controlling the llow of cryogenic fluidsunder extremely low temperature conditions.

It is a further object of this invention to provide a valve structurefor controlling lthe llow of cryogenic lluids under extremely lowtemperature conditions with minimum pressure loss to the cryogenic uidspassing through the valve.

It is a Ifurther object of this invention to provide a greatly improvedbutterfly valve for -controlling the dow of cryogenic iluids underextremely low temperature conditions insuring minimum leakage when theValve is in the closed position.

lt is a further object of this invention -to provide a greatly improvedValve structure for controlling the ow of a fluid which is suitable foruse at the extreme temt erature ranges from F. to 452 F.

It is a further object of this invention to provide a greatly improvedbutterfly valve `for controlling the flow of cryogenic fluid underextremely low temperature conditions which requires a minimum force atall times to open and close the valve regardless of the temperatures towhich the structure is subjected.

'Other objects of .this invention will be pointed out in the followingdetailed `description and claims and illustrated in the accompanyingdrawings which disclose, by way of example, the principle lof thisinvention and the best mode which has been contemplated of applying thatprinciple.

ln the drawings:

lFIGURE 1 is a iront elevational View of a preferred embodiment of thisinvention;

-F'lGURE 2 is a top sectional view of the embodiment shown in FIGURE l,taken along lines 2 2;

FIGURE 3 is an enlarged view of a portion of the apparatus shown in`FLGURE l showing the mounting and sealing arrangement for the butterilyvalve operating shaft.

ln general, the apparatus of this invention comprises a butterfly valveincluding a central, vertically positioned, rotatable shaft rigidlylcoupled to a reinforced butterliy disc member adapted to rotate from afully open, or inline position, within a cylindrical conduit to aposition at right angles thereto 'for closing on the ow of cryogenicfluid. A llexible lip seal or resilient annulus of plastic material isrigidly coupled to the outer periphery of the disc member and makescontact with a cylindrical Valve seat member embraced by Ithe conduitwhich is also formed of resilient plastic material having similarcoeicient tof expansion and low :temperature properties with bothplastic members retaining iiexibility, even at the extremely lowtemperatures to which the valve structure is subjected. The axis of thevertical support shaft is upstream slightly from the contact plane ofthe lip seal member allowing the valve to move quickly away from theseat to prevent its vibrating or sticking against the cylinder wall.Because of the differences in the expansion rate of the materials makingup the valve structure, the -butterlly valve supporting shaft is locatedin the shaft axial plane by means of Belleville-type springs allowingthe shaft, butterfly, or valve body to grow or shrink relative to anyother part wit-h no result-ant binding of parts. The structure mayinclude sealing means on either side of the anti-friction bearingssupporting the shaft within the valve housing to insure against moistureifreezing into ice within the bearing races. The valve structure justdescribed is therefore particularly adaptable to controlling the flow ofa cryogenic fluid such as liquid oxygen wherein the valve structure issubjected to extremely large temperature variation.

Referring now to Ithe drawings, the cryogenic fluid valve of the presentinvention is adapted to be used with an elongated cylindrical conduit 10formed of a metal such as steel or the like. A butterfly type of valvehas been found to require the smallest total overa-ll volume, includingmechanism, for a given free flow area and lowest pressure drop when inthe open position. The butterly valve consists in this case of atransverse shaft 12 which is mounted vertically within a short portion50 of the horizontal conduit 10. The shattt 12 is rigidly 3 coupled to adisc or butterfly member 14 which extends laterally across the conduit.The butterfly member 14 consists of a single element ymetallic disc ofcast metal construction or the like, including a series of lateralgroove portions 16 extending inward from the outer periphery of theupstream face of the butterfly member. In order to rigidly couple `thebutterfly member 14 to the vertical shaft l0, there is provided a pairof integral, vertical `flanges 1S which form a central recess 20, theflanges 1S -acting to embrace the shaft 1% within the recess 26.Suitable mounting bolts y22 extend laterally through both the flangemembers 18 and the central shaft 10, the bolts l22 including nuts 24allowing the butterliy member 14 to be rigidlyV secured to the rotatingshaft member l2 at ve different locations along .the axis of the shaft.A number of reinforcing ridges 26 are formed between the grooves'l, thealternate placement of grooves 16 and ridges 26 acting Vto provide aone-piece butterfly memberhaving vextreme rigidity regardless of thepressures exerted by the low temperature liquefied gas within theconduit 10. While butterfly valve designs are known widely for their usein systems requiring lowpressure loss as the liquid lflows through thevalve when the valve is in the fully open position, it has been foundthat valve structures of this type have a disadvantage when used inapplications where there is an extreme variation in temperature. Thevalves have a lgreat tendency to freeze in either the extreme open orclosed positions or even at points intermediate thereof. This can easilybe `seen when viewing FIGURE l which shows the valve in a closedposition wherein full peripheral contact over 360 of the peripheralsurface of the buttery member 14 would, of necessity, require equalexpansion and contraction of the stationary and moving portions of thevalve structure if metal-to-metal contact existed between the conduit 10and the buttery 14. Since there is no assurance that these members willcontract and expand at equal rates, or that the full sealing contactwill exist throughout the temperature range, the present inventionincorporates a lip seal composed of a plastic material while the innersurface of the stationary portion of the Valve forming the valve seat isprovided with a material also composed of plastic, having the samethermal rate of `expansion, to enable both the moving element and thestationary Velement to retain flexibility or resilience regardless ofthe low temperature to which the elements are subjected. Referring toVFIGURE 2 of therdrawings, the butterfly or disc member -14 includes afirst undercut, peripheral portion 2S forming an abutment 30 and asecond undercut portion 32 forming an abutment 34, the second undercutportion 32 being of less radial :distance from the peripheral edge y35of butterfly 14 than undercut portion 23. It should be noted that thediameter of the metallic butterfly disc 14 is slightly less than theinternal diameter of conduit portion 50 which is also the same diameterasV that of the valve seat member 38. Thus, at no time does theV butterydisc member 1'4 contact the valve seat 38, even when the valve Vis inthe fully closed position. The flexible lip seal 4b comprises arelatively 'thin resilient annulus of flexible plastic materialv such asKel-F, the annulus being positioned within the second undercut portion32 with Yits inner peripheral sui-face contacting the abutment 34 andits outer' peripheral surface contacting the valve seat member 3S.Suitable members such as retaining rings 42 act to hold the annular lipseal member in contact with the metallic butterfly 14. tIn order toprevent the flexible lip seal from folding back upon itself, creatingleakage, t-he lip seal is supported on the downstream side by asemi-rigid support member 44 which is positioned within the rst undercutor recesslls with its inner peripheral edgeV contacting abutment 30 andits forward port-ion including anV inwardly directed lip portion 46which tends to force the flexible sealing annulus 40 inwardly at someangle of inclination with respect to the laterally extending butterflymember 14 and the valve lseat member 38. The semi-rigid support member44 extends as close as possible to the inner peripheral surface of theseat member 38 as `is compatible with the free-turning of the valvemember l@ but is spaced a sufficient distance from the seat member 3S asto allow contact to exist only between the flexible lip seal 40 and thecooperating seat member 38.

It is advantageous to provide a valve structure where the valve membermoves as quickly away from the seal as possible during opening of thevalve. The present invention pro-vides for this requirement by allowingthe valve to be pivoted slightly upstream from the plane of of the Ilipseal. Referring to FIGURE 2, the plane of contact of the butterfly valvelies along a line passing through the butterfly valve member 14 somedistance downstream from the axis of the rotating shaft `12. In effect,this .places the contact area of the lip seal at some angle withrespectto the stationary valve seat member 33 rat-her than being perfectlyperpendicular to this mem ber when the Valve is in a fully closedposition. This allows the lip seal 40 on lthe right side of the shaft12, FIG- URE 2, whch moves downstream, to move quickly :away from thecooperating seat member 38 when the valve is opening. This acts toprevent its vibrating or chattering against the seat member 38. In likemanner, further opening of the valve automatically allows the lip seal40 Von the left side of the shaft 12, FIGURE 2,v :to move more quicklyaway from the seat member 38 Vin this area due to the ltapered portion37 of the valve seat member. The effect of the particular flexiblelipseal construction employed in the butterfly valve of this invention actsto protect the seal from deterioration and destruction during use,especially under the extreme conditions encountered in controlling theow of a cryogenic fluid.

The present invention is particularly useful for a valve which isemployed in systems operating through extensive temperature dierences.In order that the vdisc-like butterfly valve 14 may be free to'grow orShrink in relation to the Vconduit 1G, as well as to provide an eectiveseal which ywill operate under all conditions regardless of thetemperature to which the relative moving parts are subjected, there isemployed va valve seat or member 38 formed as a cylindrical seal whichmay be constructed of a suitable plastic having the desired propertiessuch as Teflon. The use of a flexible lip seal member 40 and acylindrical seal 38 which is valso made of plastic such as Teon allowsboth members to retain a certain flexibility even under low temperatureconditions. This results in reducing the friction to a minimum at thepoint of contact of the moving lip seal 40 `and the stationary valveseat member Y38 while at the same time insuring full and complete`sealing Iof the valve. In order to securely mount the Tefloncylindrical seal member 38 along `the inner peripheral su-rface ofcasing or conduit 10, there is provided a special section of thisconduit indicated generally at 50. The end of each of the conduitsections 10 includes an outwardly directed radial ange 52 which isadapted to abut a like ilange 54 formed on either end of the shortcylindrical member 56. Each pair of flanges 52 and l54 have a suitableV-shaped groove Iwithin their abutting surfaces as at 56 .andthe-.cooperating grooves receive a metal O-ring or similar sealing means58 which act to prevent radial leakage of the uid passing through theconduits 16. Suitable clamping members such as bolts 60 are provided forsecuring the shortened section 50 to the rest of the conduit sections 10in rigid relation. To achieve a leak-free sealing cylinder, the sealingcylinder'or seat member 38 is specially mounted on the cylindricalsection 50' such that the main portion of the sealing cylinder may befree to expand or contract with temperature variations. This along withthe fact that the contact portion of ythe butterfly valve is formed of alike material having similar characteristics results in extremely lowleakage regardless of the temperature to which the valve structure issubjected. The short conduit section 50 is formed with a generallylongitudinal dat surface 62 which receives the sealing cylinder and isfurther formed at the downstream end with a rst enlarged diametersection indicated generally at 68 and a second enlarged diameter sectionat 66, formed within the lower flange 54 of the conduit section 50. Acooperating annular ring member which is "formed of a metal similar tothat of conduit sections and 50 is of general L-shaped conguration incross section, including an outwardly directed base portion 72 and alongitudinally directed section 74. The annular ring member 76cooperates with the enlarged diameter sections 66 and 68 of member 50 toform a leak-free pocket for receiving the downstream end 64 of thesealing cylinder 38. At the upper end of conduit 50 there is formed,within upper flange portion 54, a longitudinal groove 76 acting toreceive the upper end 78 of the sealing cylinder 38. In this manner, themain portion of the sealing cylinder is free to expand or contract withtemperature change in either a radial or longitudinal direction,insuring low leakage regardless of the temperature to which thestructure is subjected.

`Because of the extensive temperature differences which are involved incontrolling the ilow of a liquefied gas where the ambient temperaturemay be as high as 165 F. and the temperature of the liqueed gas may beas low as 452 F., all of the elements must be free to grow or shrink inrelation to the other parts without mechanical binding. In the use of abutterfly-type valve as employed in this invention, the shaft must be somounted that the butterfly valve element 14 is free to rotate regardlessof the temperature, or temperature differences of any of the elements.The present buttery valve includes the use of support means allowing theshaft to lbe moved laterally or transverse to the ow of fluid along `thesame line as the axis of the shaft. A resilient mounting principleallows the shaft to move automatically in response to stress caused bygrowth or shrinking of the elements making up the valve structure, withthe valve member 14 freely movable at all times. The butterfly valvesupporting shaft 12 is mounted within a valve casing Si) on threeanti-friction bearings of the ball bearing types S2, S4, and 86, FlGUREl, such that the shaft is allowed to freely rotate within the casingwith the axis of rotation lying along a line drawn vertically throughthe center of the conduit section 50. The inner races of the ballbearing members S2, 84, and 86 are to float along the shaft 12, whilethe outer races are frictionally held within an upper bore S8 and alower bore 9i? formed centrally of the casing member S0. In order tomake the butterfly valve 14 and -its rotating shaft 12 free floating, apair of Belleville-type springs are positioned at each end of the shaftadjacent the ball bearing members 84 and S6. rlihe upper Bellevillespring 92 and the lower spring 94 exert `axial compressive forcestending to keep the shaft 12 centered within the conduit 1l) but theiiexure or" the springs allows the shaft to move axially for locatingthe point at which the valve is free to turn on the valve seat memberwith minimum effort, regardless of variation in temperature.

lThe bearings 82, 84, and 86 at both ends of the shaft are sealed fromcontact with the ilowing medium such as liquelied gas and the ambient bysuitable sealing means. The means employed in the upper and lower end ofthe valve structure are similar, `and reference to FIGURE 3 which showsan enlarged portion of FIG- URE 1 discloses `the means for mounting thevertical shaft within the casing member 80. The shaft 12 is locatedcentrally of casing member 8) and passes iirst through bore 9S which isof a diameter slightly greater than the diameter of shaft 12 at thispoint. A second bore 9@ is of somewhat greater diameter and is adaptedto receive the supporting ball bearing member 86. The

inner race of bearing 86 is free to move on the shaft but secured by themeans of a conventional nut 160 and 'washer 162. The shaft is allowed tomove in -an axial direction slightly in response to variations intemperature to locate the butterfly valve 14 centrally of conduit 50 bythe use of Belleville-type springs 94 which exerts an upward compressiveforce on shoulder l104 `formed ntermediate of bore 98 and bore 9i),while subjecting the ball bearing race member `86 to a downwardcompressive force on the ball bearing member 86 near :its outerperiphery. A pair of washer members 106' and 108 are positioned on eachside of the Belleville spring 94, and a sealing member 11d) is mountedin a suitable annular undercut portion 112 of the shoulder 104. The sealprevents the ingress of the low-temperature iluid such as liquefied gaspassing through the valve proper into bore 9@ and coming into contactwith the ball bearings 86. The lower end of casing member SG terminatesin `a bore 114 which is of slightly greater diameter than bore 90forming an annular recess for receiving a Teflon seal member .116. TheTeon seal member is of generally L-shaped `conliguration contacting aradial surface 118 and an axial surface 12h formed within casing 80 as aresult of enlarged bore 1-1'4. A generally dis-shaped end cap 129, whichis yformed of metal or the like, includes an inwardly directed flangeportion 122 having an external diameter slightly less than the diameterof recess 14 and a second external diameter at the extreme forwardportion of the iiange approximately the same as the Adiameter of bore90, creating a sealing pocket which acts to compress the Teon sealmember 116 within the pocket as the end cap is rigidly connected to thecasing portion 80, -t-hus sealing the bearing S6 from the ambient. Thesealing member 11G and the sealing member 116 therefore act to isolatethe bearing cavity from both the uid passing through the conduit 10 andthe ambient atmosphere. Any moisture which would normally seep into thebearing is therefore prevented from entering and 'freezing as thetemperature of ythe assembly passes from 165 =F. through 32 F. or the:freezing point of water to temperatures in the range of 452 F. Thiswould normally result in the formation of ice particles within -thebearing, either preventing rotation of `the valve or at least causingexcessive wear to the moving elements of the bearing structure.

The valve assembly is shown attached to Ia generally horizontal supportmember which may be a portion of the same means for supporting themechanism (not shown) acting lto turn the valve shaft 12 when the valveis to be opened or closed. Members coupling the casing portion 5@ to asupport member 130 are not shown but may consist of clamping memberssuch as bolts or the like. An intermediate disc member 132 acts toprovide an effective seal 'between the stationary support member 130 andthe valve proper by including an annular groove 134 adapted to receivean O-ring or other sealing means 136. The upper end of casing 8Gincludes an annular undercut section 133 forming an abutment surface1140 which acts in conjunction with undercut portion 142 of member 132to form a suitable sealing pocket for receiving a Teflon sealing member144 acting to prevent the ingress of moisture from the atmosphere intothe cavity or bore 83 carrying the twin ball bearing structures 52 and84. A second annular sealing member 146 is positioned between theBelleville-type spring 92 and a lower inwardly directed flange 148formed integrally of the casing S0 to prevent high pressure uid ifromthe conduits 10 and 56 from entering into the bearing-receiving bore 88.

With this construction, there is provided an extremely effective readilymovable, buttery valve for controlling the flow of `a liqueiied gas atextremely low temperatures requiring a minimum of expenditure of energyto open or close the valve, While obtaining low pressure loss Vthroughthe valve under full flow conditions. At the same time, the valvestructure is suitable for use at extreme ytemperature ranges from amaximum of 165 F. to a minimum of 452 IF., Wit-h minimum or zero leakagebecause of the unique use of contact surfaces formed of plastic materiallin which the flexibility of the 'materials is retained even at lowtemperatures. Even under extreme temperature changes, lwith the elementsmaking up the valve structure expanding and contracting yat dierentrates, the use of flexible mounting means allows the butterfly yvalve toshift axially relative to the stationary casing and to seek Ianequilibrium position resulting in continued minimum energy requirementsfor movement of the valve from an open to closed position, or viceversa. The present system alsotprovides an advantageous arrangement forsealing off the ball bearings supporting the rotating shaft from -bothambient and the moving iuid to prevent freezing of condensate within thebearing structure preventing interference with the rotation of the valvewithin the casing.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in the artVwithout departing tfrom the spirit of the invention. It is theintention therefore to be limited only as indicated by the scope of thefollowing claims.

What -is claimed is:

1. A butterfly-type valve, comprising a conduit dening a ow passage, acylindrical valve seat disposed within the conduit and about a planenormal to the longitudinal axis of the ow passage, a butterfly discVmember disposed within the flow passage alignable with the -valve seatand having an outer peripheral dimension smaller than the valve seat,`mounting means spaced from said plane and diametrical of the valve seatadapted to rotate the disc member to a valve closed position alignedwith the valve seat and to a valve opened position normal to said valveclosed position, a exible annulus of substantially uniform sectionsecured to the disc member and having a generally cylindrical outerperipheral dimension larger than the valve seat and seatable therewithin the valve closed position to close the flow passage, and a rigidannulus secured to the disc member on the downstream sidetof theIilexible annulus in the valve closed position, the outer peripheraldimension of the rigid annulus being smaller than the valve seat anddiverging outwardly from the disc member in an upstream direction in thevalve closed position against the ilexible annulus to slope theoverlapping iiexi'ble annulus with respect to the valve seat and to thedisc member.

2. butterfly valve, comprising a conduit deiining a ilow passage, acylindrical valve seat disposed within the conduit and about a planenormal to the longitudinal axis of the flow passage, a butterfly discmember disposed within the flow passage alignable with the valve seatand ha-ving an outer peripheral dimension smallerV than the valve seat,mounting means spaced from said plane andV diametrical of the valve seatadapted to rotate the disc member to. .a valve closed position normal tothe longitudinal axis of the valve seat aligned with the valve seatandto a valve opened position normal to said valve -closed'positiom aflexible annulus ofvsubstantially uni- 'formsection and having agenerally cylindrical outer peripheral dimension Vlarger than the valveseat and seatable therewith in the valve closed position to close theflow passage, said spaced mounting means moving said flexible annulustoward the seating position initially lin a non-tangent relationship,the outer peripheral dimension of the rigid annulus being smaller thanthe valve seat and diverging outwardly from the disc member in anupstream direction inthe valve closed position against the flexibleannulus to slope the overlapping tlexible annulus with respect to thevalve seat and to the disc mem- References Cited in the file of thispatent UNITED STATES PATENTS 2,488,380 Danks Nov. 15, 1949 2,586,927Frantz Feb. 26, 1952 2,776,104 Sinkler Jan. l, 1957 2,835,268 DillbergMay 20, 1958 2,924,424 Titterington Feb. 9, 1960 FOREIGN PATENTS 73,467lFrance Sept. 5, 1960 1,047,554 Germany Dec. 24, 1958 Y 1,125,411 FranceJuly 16, 1956 OTHER REFERENCES Chemical & Engineering News, vol. 30, No.26, June 30, 1952, pp. 2688-2691 (251-268) (copy in Scientiiic Library).

1. A BUTTERFLY-TYPE VALVE, COMPRISING A CONDUIT DEFINING A FLOW PASSAGE,A CYLINDRICAL VALVE SEAT DISPOSED WITHIN THE CONDUIT AND ABOUT A PLANENORMAL TO THE LONGITUDINAL AXIS OF THE FLOW PASSAGE, A BUTTERFLY DISCMEMBER DISPOSED WITHIN THE FLOW PASSAGE ALIGNABLE WITH THE VALVE ANDHAVING AN OUTER PERIPHERAL DIMENSION SMALLER THAN THE VALVE SEAT,MOUNTING MEANS SPACED FROM SAID PLANE AND DIAMETRICAL OF THE VALVEADAPTED TO ROTATE THE DISC MEMBER TO A VALVE CLOSED POSITION ALIGNEDWITH THE VALVE SEAT AND TO A VALVE OPENED POSITION NORMAL TO SAID VALVECLOSED POSITION, A FLEXIBLE ANNULUS OF SUBSTANTIALLY UNIFORM SECTIONSECURED TO THE DISC MEMBER AND HAVING A GENERALLY CYLINDRICAL OUTERPERIPHERAL DIMENSION LARGER THAN THE VALVE SEAT AND SEATABLE THEREWITHIN THE VALVE CLOSED POSITION TO CLOSED THE FLOW PASSAGE, AND A RIGIDANNULUS SECURED TO THE DISC MEMBER ON THE DOWNSTREAM SIDE OF THEFLEXIBLE ANNULUS IN THE VALVE CLOSED POSITION, THE OUTER PERIPHERALDIMENSION OF THE RIGID ANNULUS BEING SMALLER THAN THE VALVE SEAT ANDDIVERGING OUTWARDLY FROM THE DISC MEMBER IN AN UPSTREAM DIRECTION IN THEVALVE CLOSED POSITION AGAINST THE FLEXIBLE ANNULUS TO SLOPE THEOVERLAPPING FLEXIBLE ANNULUS WITH RESPECT TO THE VALVE SEAT AND TO THEDISC MEMBER.